1. Contribution from the Max-Planck-Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany, Institut de Physique des Nanostructures, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland, and Departments of Chemistry and Physics & Astronomy, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada